The Elements of Fire and Water
How strange it seems that the two elements that unite to produce the most important compound to life on earth--water--also united to produce that ghastly fireball at Cape Canaveral.
Hydrogen and oxygen compete with carbon as being the elements most vital to life as we know it. We seldom stop to consider that the oxygen we breathe is the most corrosive element in the universe. It combines with--rusts--almost everything. If we were to breathe pure oxygen at atmospheric pressure, it would be highly toxic. It is not just fortunate happenstance that the earth's atmosphere is diluted 79 percent by other gases (primarily inert nitrogen). That's the way the planet's atmosphere has evolved over a very long time, and we have adapted to its changes. But a creature accustomed to a methane atmosphere, for example, would probably take one breath of pure Rocky Mountain air and expire.
While oxygen is the most abundant element on earth, hydrogen is the most abundant element in the universe, and it composes almost all of the sun and stars. Very little free hydrogen is found on earth because it is so light that the atoms escape into space. But oxygen is a killer. It combines with every other element, except the inert gases, in a process called oxidation. If it happens to iron (that is, if oxygen atoms combine with iron atoms), we call it rust. If it happens to silver, we call it tarnish. If it happens to wood or paper (at a much accelerated rate), we call it flame. If it happens to hydrogen, we call it water or an explosion, depending on the circumstances.
Water is oxidized hydrogen, H2O, the combination of two atoms of hydrogen to one of oxygen. In this particular configuration, the killer aspect of oxygen is cleverly disguised. Oxygen, combined as water and then with iron as ferric oxide, may take a century to rust your Mercedes apart. In its pure form at atmospheric pressure, it may take a few hours to burn your lungs out. In the presence of hydrogen, it can alleviate your thirst as water or incinerate you in an instant in an explosion.
In 1766 the British scientist Henry Cavendish burned carefully measured amounts of hydrogen and oxygen, and found that the weights of the two gases could be completely recovered as water. At the time, this was sufficient evidence that water was composed of hydrogen and oxygen.
It was also known in the eighteenth century that sparking a mixture of hydrogen and oxygen (rather than burning the two slowly) would create a right smart snap and a few minuscule droplets of water. But the best instruments of the day were not sufficiently accurate to tell Cavendish that something was missing--not all of the gases had been converted to water. We now know that a minuscule amount of the mass was converted into a lot of energy. It remained for Einstein to tell us that energy and mass are equivalent, and that it doesn't take much matter to make an awful lot of energy. In combining hydrogen and oxygen, you get the energy to drive a spaceship, plus the water that forms the majestic plumes our shuttle launches leave behind.
The water that drips from automobile tail pipes in the winter, forms plumes of steam from power plants, or soaks the insulation in a well-insulated house without proper ventilation is created by oxygen in the air combining with the hydrogen from whatever hydrocarbon is being used for fuel.
To make a rocket engine that deliberately combines oxygen directly with hydrogen and then ignites the mixture is truly to create, as it has been described, a "controlled explosion." Because the gases combine so easily and completely, they make an ideal fuel. The trick is controlling the flow.
In our mind's eye, we tend to see that massive red tank attached to Challenger as weighing as much as it would have if it were filled with water. It wasn't really that heavy. The clear liquid hydrogen which it carried weighed only 0.07 as much as an equal volume of water, and the light blue liquid oxygen weighed only 0.89 times as much. They were supposed to have been mixed in enormous quantities in the ratio of two atoms of hydrogen to every one of oxygen and ignited in the combustion chamber. This should have produced a tremendous amount of power and the brilliant white contrail of water vapor that we have become accustomed to seeing.
But something went wrong.
